Hypo- means 'under' or 'insufficient'. The thyroid gland produces hormones that keep the metabolism functioning effectively. Hypothyroidism describes the general effects of a severely under active thyroid gland; where not enough hormones are produced to keep the body functioning properly. The adult form of this condition, which affects women five times as often as men, is also known as myxoedema.

Hypothyroidism in children can be present from birth (congenital hypothyroidism) or may develop in infancy. It is more common in adults from middle age.

Symptoms

The symptoms of adult hypothyroidism are dull facial expressions and a low-pitched and horse voice. Skin is often dry and scaly, cold, thickened and coarse. Hair may be scanty, coarse and brittle. Often eyebrows are greatly thinned or even partly missing. The affected person may be lethargic, easily fatigued, slowed in body and mind and in many cases suffers muscle aches, loss of menstruation, deafness, angina pectoris, heart failure, anaemia and constipation. The features of hypothyroidism include:

• Muscle weakness and cramps
• A hoarse voice or cry
• A slow heart rate
• Dry flaky skin
• Weight gain

Untreated childhood hypothyroidism is characterised by underdevelopment, delay in the development of teeth, delay in reaching sexual maturity may delay the development of the brain

Causes

The commonest cause of hypothyroidism is an autoimmune inflammation of the thyroid gland (thyroiditis). This can result in a severely damaged thyroid gland that functions poorly or not at all. Hashimoto's thyroiditis is a common form of the disease which runs in families.The second most common cause is medical overtreatment for hyperthyroidism (see article on Hyperthyroidism) with radioactive iodine or surgery. The damage to the thyroid gland may be permanent. Underactivity of the gland caused by drug treatment usually reverses after the treatment is complete.

Lack of enough iodine in the diet reduces the production of thyroid hormone. This causes the pituitary gland (a hormone-producing gland at the base of the skull) to produce more thyroid-stimulating hormone (TSH). The thyroid gland then works harder, becomes larger (this is visible as goitre) and traps all available iodine. Although this does not usually cause hypothyroidism, if the lack of iodine is very severe, hypothyroidism may result.

In Britain this cause of hypothyroidism is virtually unknown as even very poor diets contain enough iodine, but in some other countries there is the risk of iodine deficiency.

Hypothyroidism in children is usually caused by severe lack of iodine in the mother during the pregnancy and in the diet after birth. This has been common in countries where levels of iodine in the soil are low, but it is becoming less common since the cause has been understood and iodine has been added to domestic salt.

Treatment

Treatment for hypothyroidism is by thyroxine. In congenital hypothyroidism, the diagnosis must be made within a few days of birth and thyroid treatment started at once in order to prevent or markedly reduce mental impairment.